1. Field of the invention
The present invention relates to a developer used for an electrophotographic image forming apparatus.
2. Description of the Related Art
An example of a conventional developer is shown in FIG. 9 of the accompanying drawings. The illustrated developer includes a toner container 1, a partition 2, first and second screws 3A-3B, a conveyor roller 4, a first toner blade 40, a second toner blade 50, and two developer rollers 5A-5B.
The space within the container 1 is divided into a first portion 10A and a second portion 10B by the partition 2. The first screw 3A is provided in the first portion 10A, while the second screw 3B is provided in the second portion 10B. The first toner blade 40 is arranged adjacent to the conveyor roller 4, while the second toner blade 50 is arranged adjacent to the first developer roller 5A. The toner contains magnetic particles or carriers.
Additional toner is supplied from a toner hopper 60 into the first portion 10A of the container 1. The supplied toner is mingled with the magnetic carriers by a rotary mixer 61. Then, as shown in FIG. 10, the toner is sent in an Na-direction by the first screw 3A toward a first opening 20A (see arrow Q1) formed at one lower corner of the partition 2. Through this opening, the toner is supplied from the first portion 10A to the second portion 10B (see arrow Q2). Then, the toner is sent in an Nb-direction, opposite to the Na-direction, by the second screw 3B toward a second opening 20B (see arrow Q3) formed at the other lower corner of the partition 2. Through the second opening 20B, the toner returns to the first portion 10A of the container 1 (see arrow Q4).
Referring back to FIG. 9, the conveyor roller 4 and the two developer rollers 5A-5B are elongated generally in parallel to the second screw 3B. In operation, the conveyor roller 4 picks up the toner in the second portion 10B of the container 1, to pass it to the first developer roller 5A. At this time, an excessive amount of toner is scraped off the conveyor roller 4 by the first toner blade 40. Likewise, an excessive amount of toner passed onto the first developer roller 5A is scraped off by the second toner blade 50.
With the above arrangement, the toner in the second portion 10B of the container 1 is expected to be uniformly distributed along the entire length of the second screw 3B. Consequently, the toner maybe supplied to the first developer roller 5A uniformly over its entire length (and to the second developer roller 5B) via the conveyor roller 4. The excessive toner scraped off by the toner blade 40 or 50 will fall into the second portion 10B of the container 1, to be picked up again by the conveyor roller 4.
The conventional developer has been found disadvantageous in the following points.
For proper circulation of the toner within the container 1, the four toner transfer rates Q1-Q4 need to be well balanced. Unfortunately, however, the transferability of the toner tends to worsen with time (as the print count increases), which makes the toner circulation irregular In such an instance, the toner transfer rate Q4 through the second opening 20B may unduly be smaller than the toner transfer rate Q3 by the second screw 3B. As a result, an excessive amount of toner will accumulate around the second opening 20B in the second portion 10B of the container 1. Such an imbalance in toner distribution makes it impossible to supply toner uniformly over the entire length of the developer rollers 5-5B, which gives rise to the deterioration of the print quality. In addition, the accumulated toner may leak out of the container 1.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a developer in which toner is uniformly distributed over the entire length of a developer roller even if the transferability of the toner deteriorates with time.
According to the present invention, there is provided a developer including: a container for holding developing material; a partition dividing the container into a first chamber and a second chamber, the partition being formed with first and second openings for causing the first and the second chambers to communicate with each other, the partition being also formed with a third opening above the second opening for introducing an excessive amount of developing material from the second chamber into the first chamber; a first screw arranged in the first chamber for transferring the developing material from the second opening to the first opening; a second screw arranged in the second chamber for transferring the developing material from the first opening to the second opening; a developer roller arranged generally in parallel to the second screw and including a cylindrical surface onto which the developing material in the second chamber is supplied; a blade arranged above the second screw for scraping off an excessive amount of developing material; and adjusting means for controlling a quantity of the developing material passing through the third opening.
According to a preferred embodiment, the adjusting means may include a movable shutter for at least partially closing the third opening. In this case, preferably, the developer may further include a memory for storing data for controlling the shutter. The data to be stored in the memory may be obtained experimentally.
Preferably, the stored data may relate to the relation between the current print condition and the operation of the shutter. An example of the current print condition is the number of the performed printing operations (which may be deduced by monitoring how many times the photosensitive drum of a printer has been rotated).
Preferably, the developer may further include a sensor for detecting the developing material adjacent to the second opening in the second chamber for output of a detection signal. In this case, the shutter may be operated based on the detection signal.
According to a preferred embodiment, the developer may further include a conveyor roller for supplying the developing material to the developer roller. In this case, the adjusting means may include an additional developing material scraping blade which is arranged adjacent to the conveyor roller and movable relative to the conveyor roller.
Preferably, the additional blade may be pivotable about a predetermined axis. As the blade is caused to pivot about the axis, the clearance between the tip of the blade and the conveyor roller can be adjusted.
Preferably, the developer may further include a memory for storing data relating to a relation between a print condition and an operation of the additional blade. In operation, the posture of the additional blade is controlled in accordance with the data.